Most serpins are metastable inhibitors of serine proteinases of the chymotrypsin family; a few serpins inhibit cysteine proteinases and others mediate specific non-inhibitory biochemical events. Serpin genes are ubiquitous in the Plant Kingdom and more than 20 plant serpins from monocots and eudicots are known to be active proteinase inhibitors in vitro. Several lines of evidence suggest that highly abundant serpins associated with seed storage proteins, as well as serpins in phloem sap, may function in plant defence, but physiological roles for serpins in other plant tissues are unknown. A specific serpin isoform with the reactive-centre (proteinase-bait) sequence P2-PI? Leu-Arg-Ser/Cys/Gly is highly conserved throughout the Plant Kingdom. The inhibitory specificity in vitro of one of these 'LR serpins', recombinant BSZx from barley, has been studied in detail, revealing overlapping reactive centres at P2 Leu and P1 Arg. Intriguingly, an LR serpin appears to be the sole serpin in the unicellular green alga Chlamydomonas reinhardtii, a model organism for a range of plant processes. Microarray data from three independent studies indicates that the LR serpin from the model flowering plant Arabidopsis thaliana is significantly upregulated by cold stress, while the expression of other A. thaliana serpin genes is not similarly affected. We will present the following preliminary results: (i) Western blot analysis of A. thaliana tissues subjected to cold stress using antibodies raised against a recombinant A. thaliana LR serpin; (ii) comparison of the frost tolerance of an LR-serpin insertion mutant versus that of wildtype A. thaliana; (iii) quantitative RT-PCR data for the serpin gene in C. reinhardtii under a range of conditions including cold stress.